This invention relates to a method for producing high-purity metallic chromium and, particularly, it relates to a method for producing metallic chromium scarcely with low content level of impurities such as sulfur, nitrogen and oxygen. Metallic chromium with very low level of sulfur and oxygen contents produced by this newly proposed method provides a particularly advantageous material as chromium materials to be used in the fieled of the electronic industry and the fieled for producing the corrosion-resistive as well as heat-resistive alloys (super alloys).
Recently, metallic chromium has come to be popularly used for semi-conductors, electronic parts and dry plating. Metallic chromium containing gas such as oxygen and nitrogen at only very low level or metallic chromium with low sulfur content level is particularly advantageous for these applications.
Known methods for producing metallic chromium include the electrolytic method that decomposes Cr.sub.2 (SO.sub.4).sub.3 by applying electricity and the thermite reduction method that reduces Cr.sub.2 O.sub.3 by means of aluminum thermite reaction. However, metallic chromium by any of these known methods contains S, O, N at relatively high level and, therefore, is not good for electronic parts and super alloys, where highly pure metallic chromium is required as a constituent.
More specifically, the electrolytic method uses Cr.sub.2 (SO.sub.4).sub.3 as electrolyte and, therefore, the resultant metallic chromium contains S at a relatively high level between 200 and 300 ppm, and contains O at a level between 3,000 and 5,000 ppm and N between 200 and 500 ppm due to the use of aqueous electrolyte.
On the other hand, metallic chromium obtained by the thermite reduction method contains S at a level as high as between 200 and 400 ppm because of the fact that sulfuric acid is used for deposition of Cr.sub.2 O.sub.3 to be used as the source material and that almost all the sulfur contained in the source material remains in the resultant metallic chromium. While the O content can be decreased by increasing the rate of the reducing agent (aluminum) to be added to the source material, this in turn causes the aluminum to remain in the resultant metallic chromium at high content level. If the rate of the use of aluminum should be reduced, the O content of the obtained metallic chromium becomes inevitably as high as 3,000 to 4,000 ppm. The N content will be also as high as approximately 200 ppm.
Since metallic chromium produced by any of the known methods contains S, O and N at relatively high level, these impurities should be thoroughly removed from the metallic chromium if it be used for electronic parts and super alloys.
The vacuum carbon reduction method and the hydrogen reduction method are among the known methods for degassing metallic chromium.
With the vacuum carbon reduction method, carbon powder and, if necessary, an agglomeration agent are added to powdered crude metallic chromium and the mixture is then heated in vacuum to release the oxygen contained in the metallic chromium after turning it into CO. The hydrogen atmosphere reduction method is a method of degassing metallic chromium by heating powdered metallic chromium in an atmosphere of hydrogen and causing the oxygen contained in it to change to H.sub.2 O.
With the above described methods, where metallic chromium is heated either in vacuum, the resultant impurity content of the metallic chromium will be S.ltoreq.50 ppm for sulfur and N.ltoreq.10 ppm for nitrogen, which are by no means satisfactory for electronic parts and highly pure super alloys, where metallic chromium with a sulfur content level as low as S.ltoreq.10 ppm is required.
Besides, the vacuum reduction method that uses powdered carbon as deoxygenizing agent as described above has a disadvantage of consuming considerable time since the reduction to produce carbon monoxide takes place only after completion of the process of producing chromium carbide. It is also disadvantageous in that carbon powder and powdered crude metallic chromium can hardly be mixed evenly and, therefore, oxygen cannot be satisfactorily removed depending on the location of reaction, unprocessed carbon possibly remaining in the product.
Last but not least, the problem of safety and security is always involved in the method of hydrogen atmosphere reduction method because highly explosive hydrogen is heated to high temperature.
It is, therefore, an object of the present invention to provide a method for producing high-purity metallic chromium with low content level of impurities such as S, O and N safely and in a short period of time so that it may replace the above described carbon reduction method and hydrogen atmosphere reduction method.
Another object of the present invention is to provide a method for producing high-purity metallic chromium with low content level of impurities such as Fe, Ni and W.